Field of the Invention
The present invention relates to a capacitive micromachined ultrasonic transducer to be used as an ultrasonic transducer and the like, a method for producing the same, a subject information acquisition device, and the like.
Description of the Related Art
In recent years, various micromachined elements processed with micrometer order accuracy has been developed with the development of micromachining techniques. Using such techniques, a capacitive micromachined ultrasonic transducer (CMUT) has been actively developed. The CMUT is an ultrasonic device which vibrates a lightweight vibration membrane to transmit and receive acoustic waves, such as ultrasonic waves, and one having excellent broadband characteristics both in liquid and in the air is easily obtained. Therefore, when the CMUT is medically utilized, diagnosis whose accuracy is higher than that of ultrasonic devices containing piezoelectric elements used heretofore can be achieved. Thus, the CMUT has drawn attention as a substitute therefor. Sound waves, ultrasonic waves, photoacoustic waves, and the like are referred to as acoustic waves, which are represented by ultrasonic waves in some cases in this specification.
The capacitive micromachined ultrasonic transducer has one or more cell structures. The cell structure is configured from a first electrode disposed on a substrate of Si or the like, a second electrode disposed facing the first electrode, a cavity (gap) formed between the first electrode and the second electrode, a vibration membrane which contains the second electrode and is formed on the cavity, and a vibration membrane support portion. As one of methods for producing the capacitive micromachined ultrasonic transducer, a surface micromachining production method is mentioned which includes depositing a material on a substrate of Si or the like, and then forming the same. The formation of a cavity portion in this production method is performed by sacrificial layer etching. Specifically, a sacrificial layer is patterned in a portion serving as a cavity while leaving the size of the cavity which determines the characteristics of the transducer, and then a membrane configuring at least one part of the vibration membrane is deposited thereon. Thereafter, the sacrificial layer is removed from an etching hole (opening) which is formed in a part of the membrane and communicates with the sacrificial layer, whereby the cavity is formed. Since the capacitive micromachined ultrasonic transducer is used in a solvent in water, in oil, and the like in some cases, the etching hole provided in order to etch the sacrificial layer is sealed by depositing a film.
In the process of depositing the membrane on a sacrificial layer material, the membrane on the sacrificial layer material serves as a vibration membrane which vibrates in order to transmit and receive ultrasonic waves, and the vibration membrane in end portions of the sacrificial layer material serves as a support portion which supports the vibration membrane as a rigid body. The characteristics of the capacitive micromachined ultrasonic transducer thus produced are mainly determined based on the diameter of the cavity of the cell structure, the thickness of the vibration membrane formed on the cavity, and the height (thickness) of the cavity. In usual, when the vibration membrane vibrates, the vibration membrane is used under the conditions where the vibration membrane does not contact the bottom face of the cavity. Therefore, in order to increase the displacement of the vibration membrane for the purpose of increasing the transmission sound pressure, the height of the cavity needs to be increased. However, when the height of the cavity is increased, the thickness of the membrane support portion needs to be sufficiently secured in steps in the end portions of the sacrificial layer material patterned into the cavity shape. Therefore, the membrane needs to be formed with a film thickness which can sufficiently cover (i.e., which realizes a sufficient coverage) the sacrificial layer thickness (i.e., cavity height). The thickness is dependent also on a film forming device and needs to be at least 1 to 2 times the sacrificial layer thickness.
In view of the description above, the method including patterning a sacrificial layer in such a manner as to leave the entire shape of a cavity, forming a vibration membrane, and then removing the sacrificial layer to form a cavity has posed the following problems. More specifically, when the thickness of the sacrificial layer is increased in order to increase the transmission sound pressure, for example, for controlling the characteristics of a capacitive micromachined ultrasonic transducer, the thickness of the membrane needs to be increased in connection with the increase in the thickness of the sacrificial layer. More specifically, a large thickness of the sacrificial layer and a small thinness of the membrane have a trade-off relationship, and therefore it is not easy to increase the thickness of the sacrificial layer and reduce the thickness of the membrane.
In a capacitive micromachined ultrasonic transducer described in U.S. Pat. No. 5,894,452, a sacrificial layer is deposited on a flat substrate of Si or the like, a vibration membrane is formed thereon, and then the sacrificial layer is isotropically removed from an etching hole which opens into the center of the vibration membrane serving as the cell structure to form a cavity. According to this method, since the sacrificial layer is not patterned into a cavity shape beforehand, a step portion does not arise regardless of the thickness of the sacrificial layer. Therefore, the thickness of the membrane can be set regardless of the thickness of the sacrificial layer. However, the cavity diameter which determines the characteristics of the capacitive micromachined ultrasonic transducer is determined based on the control of the etching time of the sacrificial layer material to be removed from the etching hole which opens into the membrane, and therefore it is hard to say that it is easy to correctly control the cavity diameter. Moreover, there is a possibility that a performance variation due to a size variation of the cavity diameter in devices may arise.
As described above, according to the former production method, when the cavity height is increased, the thickness of the vibration membrane needs to be increased by a certain level or more in connection with the increase in the cavity height, which reduces the degree of freedom in design of the cell structure.